Amino acid replacements MCQs With Answer
Introduction: This quiz set helps M.Pharm students deepen their understanding of amino acid replacements and their consequences for protein structure, stability, and formulation. Questions focus on biochemical principles—side chain properties, secondary-structure propensity, disulfide bonds, deamidation, oxidation, proteolytic cleavage, glycosylation motifs—and practical formulation strategies to mitigate degradation (pH control, antioxidants, excipients). Expect items on rational substitution choices (conservative vs non-conservative), alanine scanning, and the impact of single-residue changes on aggregation, immunogenicity, and pharmacokinetics. These MCQs are designed to reinforce theoretical knowledge and to sharpen decision-making skills for protein engineering and formulation optimization in pharmaceutical practice.
Q1. Which substitution is most commonly used in alanine scanning to assess the functional importance of a residue without introducing new side-chain interactions?
- Glycine substitution
- Serine substitution
- Alanine substitution
- Valine substitution
Correct Answer: Alanine substitution
Q2. Replacement of methionine with which residue is often chosen to reduce susceptibility to oxidative degradation while maintaining hydrophobic character?
- Leucine
- Serine
- Proline
- Lysine
Correct Answer: Leucine
Q3. Which replacement is most likely to eliminate an N-linked glycosylation site (consensus sequence N‑X‑S/T) without majorly perturbing backbone conformation?
- Asn to Gln
- Asn to Asp
- Thr to Ser
- Asn to Lys
Correct Answer: Asn to Gln
Q4. Substituting a surface-exposed bulky hydrophobic residue with a charged residue is primarily intended to reduce which formulation problem?
- Protease cleavage
- Aggregation
- Disulfide scrambling
- Glycosylation heterogeneity
Correct Answer: Aggregation
Q5. Which amino acid replacement is commonly used to remove a free cysteine that causes incorrect inter‑ or intramolecular disulfide linkages while minimally altering polarity?
- Cysteine to Serine
- Cysteine to Phenylalanine
- Cysteine to Arginine
- Cysteine to Proline
Correct Answer: Cysteine to Serine
Q6. Introducing a proline residue into a loop region most directly stabilizes a protein by affecting which parameter?
- Reducing loop conformational entropy
- Increasing hydrogen bond donors
- Raising net positive charge
- Creating new glycosylation sites
Correct Answer: Reducing loop conformational entropy
Q7. Which substitution would be most appropriate to prevent deamidation at an Asn position prone to conversion to Asp during storage?
- Asn to Gln
- Asn to Asp
- Asn to Ser
- Asn to Lys
Correct Answer: Asn to Gln
Q8. Replacement of an internal glycine in a tight turn with which residue is most likely to disrupt folding because of steric clash?
- Alanine
- Valine
- Proline
- Glu
Correct Answer: Valine
Q9. Which pair of substitutions is considered conservative because both maintain a similar charge at physiological pH?
- Lysine to Glutamate
- Aspartate to Glutamate
- Tyrosine to Tryptophan
- Serine to Phenylalanine
Correct Answer: Aspartate to Glutamate
Q10. To reduce proteolytic cleavage at a susceptible Lys-Arg site, which strategic substitution is often employed without introducing a charged residue that could create a new cleavage site?
- Lys to Arg
- Lys to Pro
- Lys to Glu
- Lys to Ser
Correct Answer: Lys to Pro
Q11. Which replacement is most likely to affect a salt bridge that stabilizes tertiary structure?
- Leu to Ile
- Asp to Asn
- Phe to Tyr
- Gly to Ala
Correct Answer: Asp to Asn
Q12. Substituting tyrosine with phenylalanine removes which chemical functionality that can influence activity or post‑translational modification?
- Hydroxyl group
- Amino group
- Sulfhydryl group
- Carboxyl group
Correct Answer: Hydroxyl group
Q13. Which substitution is most likely to increase the isoelectric point (pI) of a protein if introduced at multiple surface positions?
- Aspartate to Asparagine
- Glutamate to Lysine
- Serine to Threonine
- Tryptophan to Tyrosine
Correct Answer: Glutamate to Lysine
Q14. When designing a therapeutic protein to reduce immunogenic epitopes, what type of substitution is typically considered to reduce T‑cell recognition while preserving function?
- Non-conservative substitution with bulky aromatic residue
- Conservative substitution guided by sequence homology to human proteins
- Random mutagenesis across the entire protein
- Introducing multiple charged residues in active site
Correct Answer: Conservative substitution guided by sequence homology to human proteins
Q15. Which replacement is a common strategy to prevent methionine oxidation in formulations where oxidative stress is difficult to control?
- Methionine to Isoleucine
- Methionine to Serine
- Methionine to Arginine
- Methionine to Aspartate
Correct Answer: Methionine to Isoleucine
Q16. If a substitution increases local beta‑sheet propensity at an aggregation-prone region, which residue replacement likely contributed to that change?
- Serine to Valine
- Valine to Glycine
- Proline to Glycine
- Alanine to Proline
Correct Answer: Serine to Valine
Q17. In a strategy to stabilize an antibody variable region, introducing a salt bridge typically requires substitution of residues to which complementary pair?
- Glycine and Proline
- Lysine and Glutamate
- Phenylalanine and Tyrosine
- Asparagine and Glutamine
Correct Answer: Lysine and Glutamate
Q18. Which substitution is most useful to block a serine protease cleavage site without drastically altering backbone conformation?
- Serine to Threonine
- Serine to Alanine
- Serine to Proline
- Serine to Aspartate
Correct Answer: Serine to Proline
Q19. Which computational scoring matrix is commonly referenced to evaluate whether an amino acid replacement is evolutionarily conservative?
- Hertzberg matrix
- BLOSUM matrix
- Miller index
- Kd binding matrix
Correct Answer: BLOSUM matrix
Q20. To reduce deamidation and isoaspartate formation at an Asn‑Gly motif, which design change is commonly implemented in a therapeutic protein?
- Replace Gly with Proline
- Replace Asn with Aspartate
- Replace Asn with Gln
- Replace Gly with Valine
Correct Answer: Replace Asn with Gln
